beta-Ti-25 at%Nb-xSn alloys with several Sn substitutions (x < 12.5 at%) were studied by ab initio calculations and compared with the Ti-25.05 at%Nb-2.04 at%Sn rods synthesized by copper mold suction casting, annealed and water quenched. Ab initio and XRD data agree that upon Sn substitution the unit cell volume increases almost linearly. It came out that in the case of Ti-25.05 at%Nb-2.08 at%Sn, the dopant's s-electrons introduce low energy states (around -8 eV) with anti-bonding characteristics with the first and second neighbouring atoms, in line with the experimental findings suggesting lowering of Young modulus. At high Sn contents, first or second neighbourhood may include Sn-Sn pairs that exhibit strong direction bonds at even lower energy states, thus resulting in increase of the Young modulus. These findings, in conjunction with the calculated electronic density of states suggest that the beta-phase is stable at low Sn substitutions (<6.25 at%), while higher Sn concentration (x > 12.5 at%) induce instability. These results could be of use in the design of ternary low rigidity biocompatible TiNbSn alloys.